In the fourth season episode of The X-Files, "Leonard Betts," a man is decapitated in an accident, and his headless body breaks out of the morgue and regenerates its head. In her examination of Leonard's brain, Scully finds that he is riddled with cancer: literally, all of his brain cells are cancerous. Mulder theorizes that Leonard is regenerating body parts after finding his bathtub full of providone iodine, which scientists use to aid salamanders with the regeneration of limbs. Scully tells Mulder in no uncertain terms that while amphibians can regenerate limbs, the same ability has never been attributed to any mammal, let alone a human being.

"While Scully is accurately relating what was known at the time the episode was written," Simon writes, "she probably wouldn't make such a sweeping statement today."

Scientists have long been studying amphibians in order to replicate their regenerative abilities in humans. In most species, cells start out unspecialized at the beginning of development, become specialized, and then stay in that specialization until they die. Amphibians are unique in that their cells can revert back to their unspecialized status in order to become whichever kind of cell is needed after an injury, allowing them to self-repair.

After "Leonard Betts" came out in 1997, scientists successfully demonstrated that certain mammalian species are capable of similar feats. A study with mice showed that immune-compromised mice were able to regenerate ear tissue, and even 75% of a severed tail. Experiments confirmed that immune T-cells were preventing genes that allow for self-repair to function. So theoretically, a human could regenerate if her T-cells were turned off, but since T-cells are the main weapon against tumors, she would die early in life.

"It's tantalizing to imagine a day when science can alter cells near a damaged spinal cord or amputated limb such that regeneration and a functional immune system are not at odds with each other. No one would fear amputation because the limb would just grow back. Heart muscles damaged by disease could re-form like new, and spinal cord injuries would no longer confine victims to wheelchairs.

So what does this mean for Leonard Betts? It makes some sort of sense that he would be riddled with tumors, but then he would be dead (and there's no scientific basis for ingesting tumors, as far as I can tell). In order for all of his cells to become cancerous, he would need to have inherited two faulty versions of the p53 gene, which is responsible for suppressing tumors. Mice with this condition die a few weeks after birth, riddled with tumors.

"Only fictional people like Leonard could survive with two faulty versions of the p53 gene in every cell."

As a result, it's much more "likely" that Leonard doesn't have cancer at all, but that Scully was mistaking blastema, masses of unspecialized cells that are capable of regeneration, for tumors. Blastema are found in amphibians that can regenerate (and the mice who regrew their tails), and as speculative science goes, it's not unimaginable that a human would be able to grow back limbs or appendages, like the thumb that Betts regenerates later in the episode.

"While regenerating limbs or a spinal cord may one day be science and not science fiction, regenerating a head, as Scully points out, is another issue entirely."

When Scully is skeptical that a human being can re-grow a head, Mulder points out that worms have been known to regenerate their heads when their bodies are severed. But even if humans ever achieve regenerative abilities, they would only be able to recover from injuries that didn't result in fatal blood loss and/or brain death before the damage could realistically be repaired. And further, worms are very different from humans (if that needs to be said). 20% of their bodies are composed of neoblasts, a type of blastema that remains in an immature state, ready to start dividing and specializing whenever needed. So long as a neoblast cell is present in the severed part of the worm, it can regenerate the rest of its body, even its head.

"Scully is correct to remind Mulder that while worms can regenerate their heads, Leonard Betts is no worm."